Electrical power cables typically have an outer jacket, or sheath, that surrounds the exterior of the cable and provides thermal, mechanical, and environmental protection for the conductive elements within. Outer jackets often comprise polyethylene, polyvinylchloride, or nylon.
Cables designed for medium voltage distribution (generally 5 kV through 46 kV), such as feeder cables or those designed for residential or primary underground distribution, generally have a non-expanded polymeric jacket formed in a single layer. These cables may also include elements, wires or flat straps, for example, formed within the jacket and arranged concentrically around the cable's axis and helically along its length. These elements, also called “concentric neutrals” or “wire serves,” provide a return current path to accommodate faults. The elements typically need to have the capacity to carry high electrical currents (thousands of amperes) for a short duration (60 cycles/second or less) during an emergency condition until a relay system can interrupt the distribution system.
FIG. 1 is a traverse cross-sectional diagram of a conventional concentric neutral element cable. The cable 100 contains a conductor 110, a semi-conducting conductor shield 115, an insulation layer 120, an insulation shield 125, an outer jacket 130, and concentric neutral elements 150. The concentric neutral elements 150 serve as a neutral return current path and must be sized accordingly. The insulation shield 125 is usually made of an extruded semiconducting layer that surrounds the insulation layer 120. The conductor 110 serves to distribute electrical power along the cable 100.
Jackets for concentric neutral cables are typically extruded under pressure during cable manufacture. This process, known as “extruded to fill,” leads to an encapsulated thermoplastic polymer layer surrounding the cable. Pressure extrusion causes the polymeric material to fill the interstitial areas between and around the neutral elements. Further, the materials typically selected for such processing, such as a polyethylene, have a tendency to shrink-down after extrusion and thus maintain a firm hold over the cable core. Additionally, the use of extruded-to-fill polymeric jackets are commonly employed to provide good hoop-stress protection, to lock-in the concentric neutrals, withstand reasonable temperatures during fault situations, and to provide good mechanical protection. Indeed, jackets in underground residential distribution must be robust enough to handle the mechanical rigors of installation via direct burial trenches or plow-in.
While extruded-to-fill outer jackets provide certain advantages as noted above, such outer jacket construction creates a number of issues as well. For example, a significant degree of physical force is required to remove the outer jacket from the core, increasing the likelihood of damaging the core. Indeed, in removing the jacket in the field, it is common practice for utility linemen to retrieve one of the heavy concentric neutral elements under the jacket and use it as a ripcord to pull through the jacket. The wire is lifted and pulled at an approximate 150 angle to the axis of the cable, cutting the jacket along the spiral axis of the neutral element. The force required to pull the element can be significant.
The high degree of physical force to remove the jacket arises for a number of reasons. First, due to the affinity of polyethylene class of jackets to the class of materials normally employed as semi-conducting insulation shields, there is a tendency for the two materials to stick together or form a light to moderate bond. To overcome this bonding, cable manufacturers often apply, for example, talc/mica to allow easy separation of the two layers. Water-swellable powder may also be applied as described in U.S. Pat. No. 5,010,209. The use of these powders decreases the likelihood of water migration between jacket- and insulation shield interface, in the event water enters due to a breach in the outer jacket. Second, a high degree of force in stripping or removing the jacket arises because, in encapsulating the concentric neutral elements, the jacket is often thicker than jackets in comparable cables without concentric neutrals. More than 90% of concentric neutral cables for underground residential distribution have neutral elements that range between #14 AWG (64.1 mils or 1.29 mm in diameter) to #8 AWG (128.5 mils or 3.26 mm in diameter). Industry standards often specify the minimum thickness for the jacket in such cables to be determined according to the thickness over these concentric neutral elements, resulting in a larger and more rugged jacket.
The increased size of jackets in concentric neutral cables may also cause those cables to be less flexible. Although a cable designer can specify alternate types of insulation to improve flexibility without sacrificing reliability, the overall encapsulated jacket maintains significant influence over the flexibility of such cables. Alternate jacket materials that improve flexibility are available; those materials may be undesirable because they do not satisfy more significant attributes in the cable design.
In addition, a concern in the industry exists with undesirable indentations in the insulation shield that can arise in concentric neutral cables having extruded-to-fill jackets. These indentations occur as the rigid, conventional jackets shrink down after extrusion and force the neutral elements into the shield. The indentations may increase after applying the cable to a shipping reel where the weight of the cable on the inner wraps of the reel may further induce compression. The indentations in the insulation shield take the helical path of the neutral elements. Should water enter the cable due to a breach in the jacket, the helical indentations can provide conduits or channels for the water to migrate longitudinally along the cable. At times, the indentations may transfer through the insulation shield and leave indentations to a lesser extent on the surface of the insulation.
Despite these issues, jackets for concentric neutral cables tend to be a single, encapsulated layer of polyethylene-class material to ensure that the cable can withstand the mechanical rigors of underground installation. For other types of cables, however, jackets incorporating an inner layer of expanded polymer material have been disclosed in the art to help protect cables against accidental impacts. Expanded polymeric materials are polymers that have a reduced density because gas has been introduced to the polymer while in a plasticized or molten state. This gas, which can be introduced chemically or physically, produces bubbles within the material, resulting in voids. A material containing these voids generally exhibits such desirable properties as reduced weight and the ability to provide more uniform cushioning than a material without the voids. The addition of a large amount of gas results in a much lighter material, but the addition of too much gas can decrease some of the resiliency of the material.
U.S. Pat. No. 6,501,027, for example, describes a coating layer preferably in contact with the cable sheath for providing impact resistance for the cable. The coating layer is made from an expanded polymer material (i.e., a polymer that has a percentage of its volume not occupied by the polymer but by a gas or air) having a degree of expansion of from about 20% to 3000%.
Applicants have observed that expanded polymeric materials are potential candidates for improving the structure and performance of cables having embedded elements in their jackets, such as concentric neutral power cables. Applicants have further observed that unlike conventional designs for concentric neutral cables, cables having multiple layer jackets including a layer of expanded polymeric material may result in a jacket that is easier to strip, has increased flexibility, and decreased incidence of indentations in the insulation.